Zsm-5, its preparation and use in ethylbenzene dealkylation

ABSTRACT

A new configuration of ZSM-5 is provided whereby the crystals have a higher average silica to alumina ratio at the edges of each crystallite than in the centre as determined from a narrow slit line scan profile obtained from SEM/EDX or TEM/EDX elemental analysis. Such ZSM-5 crystals are obtained by a preparation process using L-tartaric acid. The new configuration ZSM-5 provides significantly reduced xylene losses in ethylbenzene dealkylation, especially when combined with silica as binder, and one or more hydrogenation metals selected from platinum, tin, lead, silver, copper, and nickel.

This present application is a divisional of US Divisional applicationSer. No. 13/922,328, filed Jun. 20, 2013 which is a divisional of U.S.application Ser. No. 12/677,009, filed May 4, 2010, granted Nov. 5, 2013as U.S. Pat. No. 8,574,542, which application was the National Stage ofInternational Application PCT/EP08/61967, filed Sep. 10, 2008, whichclaims the benefit of U.S. Provisional Application No. 60/971,130, filedSep. 10, 2007 and claims the benefit of U.S. Provisional Application No.61/080,823, filed Jul. 15, 2008.

FIELD OF THE INVENTION

The present invention relates to ZSM-5, its preparation, catalystcomposition containing it, and its use in ethylbenzene dealkylation.

BACKGROUND OF THE INVENTION

Ethylbenzene is one of the aromatic hydrocarbons that is obtained fromnaphtha pyrolysis or in reformate. Reformate is an aromatic productgiven by the catalysed conversion of straight-run hydrocarbons boilingin the 70 to 190° C. range, such as straight-run naphtha. Suchhydrocarbons are themselves obtained by fractionation or distillation ofcrude petroleum oil, their composition varying depending on the sourceof the crude oil, but generally having a low aromatics content. Onconversion to reformate, the aromatics content is considerably increasedand the resulting hydrocarbon mixture becomes highly desirable as asource of valuable chemicals intermediates and as a component forgasoline. The principle components are a group of aromatics oftenreferred to as BTX: benzene, toluene, and the xylenes, includingethylbenzene. Other components may be present such as their hydrogenatedhomologues, e.g. cyclohexane.

Of the BTX group the most valuable components are benzene and thexylenes, and therefore BTX is often subjected to processing to increasethe proportion of those two aromatics: hydrodealkylation of toluene tobenzene and toluene disproportionation to benzene and xylenes. Withinthe xylenes, para-xylene is the most useful commodity and xyleneisomerisation or transalkylation processes have been developed toincrease the proportion of para-xylene.

A further process that the gasoline producer can utilize is thehydrodealkylation of ethylbenzene to benzene.

Generally, the gasoline producer will isolate BTX from the reformatestream, and then subject the BTX stream to xylene isomerisation with theaim of maximising the para-xylene component. Xylene isomerisation is acatalytic process; some catalysts used in this process have the abilitynot just to isomerise xylenes but also simultaneously to dealkylate theethylbenzene component. Normally the para-xylene is then separated outto leave benzene, toluene (unless toluene conversion processes havealready been applied) and the remaining mixed xylenes, includingethylbenzene. This BTX stream can either be converted by transalkylationto increase the yield of xylenes by contacting with a heavierhydrocarbon stream or can be converted by dealkylation to eliminateselectively ethylbenzene and to increase the yield of benzene, whileallowing the xylenes to reach equilibrium concentrations. The latterprocess is the subject of the present invention.

In ethylbenzene dealkylation at this latter stage of BTX treatment, itis a primary concern to ensure not just a high degree of conversion tobenzene but also to avoid xylene loss. Xylenes may typically be lost dueto transalkylation, e.g. between benzene and xylene to give toluene, orby addition of hydrogen to form, for example, alkenes or alkanes.

It is therefore the aim of the present invention to provide catalyticmaterials that will convert ethylbenzene to benzene with a reducedxylene loss.

For the conversion of BTX streams to increase the proportion of closelyconfigured molecules, a wide range of proposals utilizing zeoliticcatalysts have been made. One common zeolite group utilized in thedealkylation of ethylbenzene is the MFI zeolites and in particularZSM-5. The ZSM-5 zeolite is well known and documented in the art.

Many preparation routes have been proposed that provide active MFIzeolites, including ZSM-5, see for example U.S. Pat. No. 3,702,886.

U.S. Pat. No. 4,511,547 proposes a general preparation route for theproduction of crystalline aluminosilicate zeolites which comprisesstirring, whilst heating, an aqueous reaction mixture containing asilica source, an alumina source, an alkali source and an organiccarboxylic acid which does not contain an aromatic ring, suitably anorganic carboxylic acid having from 1 to 12 carbon atoms. The examplesof U.S. Pat. No. 4,511,547 utilise tartaric acid and, from the XRDpattern provided, produce ZSM-5 type zeolite.

Tartaric acid has two chiral centres and exists in four mainenantiomeric forms: racemic, meso, levorotatory and dextrorotatory. Theracemic form (DL-tartaric acid) is readily available and producedcommercially in Europe, South Africa and Japan while the dextrorotatoryform (L-tartaric acid) is the commercial product in the USA approved bythe FDA for use in the food and pharmaceutical industries (see theKirk-Othmer Encyclopedia of Chemical Technology, 4^(th) Edition, Volume13, pages 1071 to 1078). U.S. Pat. No. 4,511,547 is silent as to theform of tartaric acid utilized but the examples come fromJapanese-originating research and it is reasonable to conclude that theracemic form of tartaric acid was used.

SUMMARY OF THE INVENTION

We have now found that a catalyst composition containing a particularconfiguration of ZSM-5 crystals can provide significantly reduced xylenelosses in ethylbenzene dealkylation. The ZSM-5 configuration is obtainedwhen utilising only one isomer of tartaric acid in the zeolitesynthesis. The new configuration of ZSM-5 is detectable at thecrystalline level and we have developed a new method of analysingspecific X-ray spectroscopy data to enable selection of the higherperforming ZSM-5 crystals by pinpointing this new configuration.

Accordingly the present invention provides ZSM-5 crystals which have ahigher average SAR at the edge of each crystallite than at the centre,as determined via SEM/EDX or TEM/EDX elemental analysis.

Also provided by the present invention is a process for the preparationof ZSM-5 crystals of the present invention which comprises synthesizingthe crystals from an aqueous reaction mixture comprising an aluminasource, a silica source, an alkali source, and L-tartaric acid, or awater-soluble salt thereof.

Additionally provided by the present invention is a process forselecting ZSM-5 crystals having a high selectivity for ethylbenzenedealkylation, which comprises subjecting ZSM-5 crystals to SEM/EDX orTEM/EDX elemental analysis, calculating the SAR across the crystals andselecting those crystals which exhibit a U- or dish-shaped narrow slitline scan profile on a graph of SAR across the crystallite from edge toedge, wherein the average SAR is higher at the edge of the crystallitethan at the centre.

A catalyst composition which comprises ZSM-5 crystals of the presentinvention is further provided, and also an ethylbenzene dealkylationprocess which comprises contacting, in the presence of hydrogen, afeedstock which comprises ethylbenzene with a catalyst composition ofthe present invention.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a graph of the narrow slit line silica-to-alumina ratioprofile for a ZSM-5 zeolite crystallite sample.

FIG. 2 is a graph of the narrow slit line silica-to-alumina ratioprofile for another ZSM-5 zeolite crystallite sample.

FIG. 3 are plots of the profile slopes against xylene losses for variouscrystallite samples.

DETAILED DESCRIPTION OF THE INVENTION

The ZSM-5 crystals of the present invention have been found to providemuch reduced xylene losses compared with those having no differentiationin SAR from crystal edge to centre, and those prepared utilisingDL-tartaric acid. Since the reduction of xylene losses is an indicationof increased selectivity of reaction, there is accordingly an unexpectedcorrelation between the intraparticle distribution of aluminium in aZSM-5 type zeolite and the selectivity of a catalyst based on thiszeolite in the dealkylation reaction of ethylbenzene.

We have found that from the elemental maps produced from the twodimensional recording of the energy dispersive X-ray spectroscopy (EDX)coupled with transmission or scanning electron microscopy (TEM or SEM,respectively), it is possible to generate a consistent high-resolutionone dimensional profile for the distribution of specific elements. Wehave developed a new method for the generation of “narrow slit”profiles, where the lateral resolution of the profiling is kept in therange of 50 nm; this can be slightly improved if necessary. Thishigh-resolution distribution profile, e.g. across a solid particle, canbe used for the display of amounts of elements of components against adirection, from e.g. the edge to the centre of the particle. Theelemental information (including the obtained profiles) then allows thecalculation of e.g. ratios of elements or components, often used in thecharacterization of chemical substances, in a similar fashion.

In zeolite characterization the ratio of silica to alumina (SiO₂/Al₂O₃herein ‘SAR’) is an important parameter. This parameter is inverselyrelated to the acid site density associated with the presence ofaluminium in the framework of a crystalline aluminosilicate zeolite. Inhigh-resolution micrographs of SEM/EDX or TEM/EDX, the available highdata-density allows a more detailed analysis of the zeolitecrystallites. This is to say that not only an average chemicalcomposition can be obtained, but also the variations in the localchemical composition to a very high detail with ˜50 nm lateralresolution.

Conventionally SAR is determined for crystalline aluminosilicatezeolitic materials by bulk elemental analysis. As noted above in thepresent invention the SAR at crystalline level is determined utilisingelemental maps produced from energy dispersive X-ray spectroscopy (EDX)coupled with transmission electron microscopy (TEM) or scanning electronmicroscopy (SEM). EDX, TEM and SEM are documented techniques, seeChapter 9, Scanning Electron Microscopy and X-ray Microanalysis, PlenumPress, 1992, and Surface Characterization: A User's Sourcebook, editedby D. Brune, et al., Wiley-VCH Verlag GmbH, November 1997, ISBN3-527-28843-0, particularly pages 109 to 288.

The chemical composition of samples studied is often determined inscanning or transmission electron microscopy. This determination isachieved by detecting the X-rays that are produced as a result ofin-elastic collisions between the primary electrons from the SEM and theelectrons in the sample itself. Each element produces X-rays withcharacteristic energies. By measuring the energies of the X-rays emittedfrom the sample, it is possible to determine the elements present in thesample.

The strong interaction between electrons and solids means that theminimum lateral resolution possible is around 1-5 microns, depending onthe initial energy of the primary electrons. As well as a qualitativeexamination, it is also possible to quantify the results by comparingthe number of X-rays detected. This requires a number of correctionsrelating to the differences in ionization cross-section for the elementsand energy dependent absorption of the X-rays. The details of thecorrection method is not considered here but in general the accuracy ofthis analysis method is 10% or better for samples with an atomic numbergreater than 10. In the case of elements with an atomic number less than10, the accuracy can be significantly worse.

The chemical information can be measured with the beam stationary or asit is scanned. In the stationary mode it is possible to count for a fewhundred seconds which will give a spectra with a good signal to noiseratio and can be used for accurate quantification. In the scanning mode,the data is more qualitative but it is possible to build up a picture ofthe spatial distribution of the various elements in the sample. Suchscans are usually referred to as X-ray maps.

In high-resolution micrographs of SEM/EDX or TEM/EDX the high datadensity available allows further processing of the information. Itallows one to ‘look inside’ an individual zeolite crystallite, and toobtain local information about the variations in the chemicalcomposition within the particle or the crystallite itself.

Next to the standard statistical methods applied to deal with largeamounts of data, we herein propose a new method that allows averagingwithout losing the lateral resolution of the original X-ray map.

The SEM/EDX (or TEM/EDX) maps are obtained as follows. At smallmagnification, a suitable region of the sample is selected where somecrystallites are standing free from the rest of the sample. Afreestanding crystallite is selected randomly. The magnification is thenincreased so that the crystallite fills at least 75% of the viewfinderof the microscope. This is done to “maximize” the possible informationto be gathered from the elemental analysis per image taken, and toincrease the number of analysis points across the zeolite crystal.

The zeolite crystallite is then aligned so that the longest edge of thecrystal is parallel to the longer edge of the viewfinder (or the picturethat will be taken). As ZSM-5 crystallites tend to have an elongated(often coffin shaped) form, it frequently occurs that part of thecrystallite is left out of the image whilst trying to increase themagnification to occupy at least 75% of the visible area. This is to saythat in almost all cases part of the zeolite crystal has to be left outfrom the final image. It is important that one end of the crystallite isstill visible in the viewfinder of the instrument. If the alignment andthe magnification are carried out correctly, the above described processresults in a picture where both the two sides along the longest axis,and one end of the crystallite are visible in the final elemental map.

An EDX grid of sufficiently high resolution is placed on the image(typically dictated by the instrument), and an EDX elemental map isobtained which provides at each location or point the amount of eachelement at that point—a spacial composition map. From the elemental map,the SAR values at each point are calculated and a SAR map obtained. Wewill refer to both the obtained elemental and the calculated SAR map asan X-ray map, or analysis (mapping) matrix below reflecting itsnumerical nature.

The “narrow slit” line scan profiles are calculated from the obtainedX-ray SAR mapping data. The orientation of the maps is identical, so the“base” of the maps, where the zeolite crystal extends beyond the map, isat the identical edge of all pictures. Furthermore, since extra care istaken to align the crystallites along the measurement area, theorientation of the zeolite crystal is also parallel to the obtainedanalysis matrix. The realigned mapping matrices (obtained by rotatingclock- or counterclockwise) are averaged in each column over a selectedrange of rows aiming at the middle of the crystal stem, away from thebase, and from the tip as well. Approximately 30 rows are selected, andaveraged for each lateral point (column).

The X-ray maps where the crystal does not touch any of the sides arehandled differently. In this case, the middle section of the crystal isselected, averaging also approximately 30 lines.

By obtaining the ‘vertical’ averages while moving across the crystalfrom one side to the other, the averages provide a narrow slit overviewof the crystallite. It does not compromise the lateral resolution acrossthe crystallite, but still averages the local possible imperfectionswith the neighbouring map points. It is important to keep in mind thatthe averaging occurs in the direction where structural similarity (e.g.similar SAR) is expected, and not in the direction where the variationsin the composition are claimed. The obtained profiles therefore are moresensitive to structural variations in the aluminum and silicondistribution from edge to centre, while it is less sensitive toimperfections in the shape of the crystallites along the long axis.

One has to realize the very high resolution of these line scans. Thecell size is ˜50 nm that provides excellent resolution at the scale ofindividual crystallites, in the range of 1-5 μm—far beyond the normalline scan resolution obtained in e.g. extrudate analysis. It has to beremembered that this is not the native resolution of the maps, as thedata is first grouped due to various reasons. The main reason is thatthe grouping increases the signal to noise ratio in the X-ray maps, andit allows a far better detection of the elements present in only smallquantities (close to the detection limit of the equipment).

The obtained line scan profiles can be compared by fitting a secondorder polynomial to the available experimental data in the range betweenthe two maxima at the edge of the profiles. The slope of the polynomialy=a₂x²+a₁x+a₀ is determined by derivation according to the conventionsas y′=2a₂x+a₁. The slope is determined at x=0, so it is y′₀=a₁. As a₁tends to have a negative value, −y′₀=−a₁ is shown in FIG. 3 herewith.

This value can be correlated with e.g. various performance parametersmeasured with catalysts made from the zeolites studied with SEM/EDX orTEM/EDX.

In the present invention, ZSM-5 crystals are provided that have a higherlocal average SAR at the edge of each crystallite than at the centre asdetermined from a narrow slit line scan profile obtained from SEM/EDX orTEM/EDX elemental analysis as hereinbefore described. Most suitably theratio of the average SAR at the edge of each crystallite to the averageSAR at the centre of the crystallite is at least 1.15, preferably atleast 1.25. The ratio is most suitably at most 3, preferably at most 2.

The average SAR at the centre of the crystallite is the average of the 3middle SAR values of the narrow slit line scan profile, i.e. the averageof the middle value and the values either side of the middle value. Theaverage SAR at the edge of each crystallite is obtained by calculatingthe average of the SAR at the three outermost points at each end of thenarrow slit line scan profile and then calculating the average of thetwo values obtained.

Where the data is converted to a second order polynomial, then the slopefitted to the SAR values between the two edge SAR maxima, expressed as−y′₀, is most suitably at least 2, and preferably at least 3. The slopeis most suitably at most 6, and preferably at most 4.

The preparation of the advantageous ZSM-5 crystals of the presentinvention is via the process of U.S. Pat. No. 4,511,547, the contents ofwhich are hereby incorporated by reference, with the amounts of silica,alumina, alkali, and tartaric acid adapted as necessary to produceZSM-5. This adaptation would be within the normal knowledge and skill ofthe skilled person in the art. As noted in U.S. Pat. No. 4,511,547, thereaction mixture is desirably kept in a homogeneous state duringcrystallization by suitable agitation of the reaction mixture.

L-tartaric acid, or a water-soluble salt thereof, is used in thepreparation process of the present invention. This L-isomer is thedextrorotatory form and is variously named (2R,3R)-(+)-tartaric acid, or(2R,3R)-2,3-dihydroxybutanedioic acid, or (R—R*, R*)-tartaric acid,depending on the naming convention used.

We have found an unexpected correlation between the above-describedslope of the silica to alumina ratios in a crystallite ZSM-5 zeolite(MFI) structure and the xylene losses in ethylbenzene (EB) dealkylationprocess particularly when incorporated into a silica-bound catalyst,which contains 40 wt % of the above mentioned ZSM-5 zeolite in thecarrier.

Silica is preferably used as the binder in the catalyst composition ofthe present invention and may be a naturally occurring silica or may bein the form of a gelatinous precipitate, sol or gel. The form of silicais not limited and the silica may be in any of its various forms:crystalline silica, vitreous silica or amorphous silica. The termamorphous silica encompasses the wet process types, includingprecipitated silicas and silica gels, of pyrogenic or fumed silicas.Silica sols or colloidal silicas are non-settling dispersions ofamorphous silicas in a liquid, usually water, typically stabilised byanions, cations, or non-ionic materials.

The silica binder is preferably a mixture of two silica types, mostpreferably a mixture of a powder form silica and a silica sol.Conveniently powder form silica has a surface area in the range of from50 to 1000 m²/g; and a mean particle size in the range of from 2 nm to200 μm, preferably in the range from 2 to 100 μm, more preferably 2-60μm especially 2-10 μm as measured by ASTM C 690-1992 or ISO 8130-1. Avery suitable powder form silica material is Sipernat 50, a white silicapowder having predominately spherical particles, available from Degussa(Sipernat is a trade name) A very suitable silica sol is that sold underthe trade name of Bindzil by Eka Chemicals. Where the mixture comprisesa powder form silica and a silica sol, then the two components may bepresent in a weight ratio of powder form to sol in the range of from 1:1to 10:1, preferably from 2:1 to 5:1, more preferably from 2:1 to 3:1.The binder may also consist essentially of just the powder form silica.

Where a powder form of silica is used the binder in the catalystcomposition of the present invention, preferably a small particulateform is utilised, which has a mean particle size in the range of from 2to 10 μm as measured by ASTM C 690-1992. An additional improvement incarrier strength can be found with such materials. A very suitable smallparticulate form is that available from Degussa under the trade nameSipernat 500LS.

The silica component used may be pure silica and not as a component inanother inorganic oxide. For certain embodiments, the silica and indeedthe carrier, is essentially free of any other inorganic oxide bindermaterial, and especially is free of alumina. Optionally, at most only amaximum of 2 wt % alumina, based on the total carrier, is present.

For certain embodiments a surface modification treatment may beperformed. For such embodiments, the presence of alumina candetrimentally affect the physical integrity of the carrier and so isless preferred.

The ZSM-5 zeolite can exist in various forms depending on the ionpresent at the cation sites in the zeolite structure. Generally theavailable forms contain an alkali metal ion, an alkaline earth metalion, or a hydrogen or hydrogen precursor ion at the cation site. In thecatalyst composition of the present invention, the zeolite is present inthe form containing hydrogen or hydrogen precursor; this form iscommonly known as the H⁺ form. The zeolite may be used either in atemplate-free or a template-containing form.

The bulk overall SAR, measured by bulk (rather than crystalline level)elemental analysis, of the ZSM-5 is preferably at least 25, mostpreferably at least 30, and is preferably at most 100, most preferablyat most 90, especially at most 50.

The ZSM-5 zeolite can exist in a number of particle size ranges.Suitably the zeolite has a primary particle diameter in the range offrom 20 nm to 10 μm. Useful catalysts have been prepared using a largecrystal size ZSM-5 zeolite having an average crystallite size in therange of from 1 to 10 μm, and also using a small particle size ZSM-5having a primary particle diameter below 200 nm Generally, in terms ofparticle size distributions, the ZSM-5 may have a particle sizedistribution in which the diameter of 50% of the particles, D(v, 0.5),is greater than 2 μm and that of 90% of the particles, D(v, 0.9), isless than 30 μm.

The zeolite is an important factor in the activity and selectivityproperties shown by the catalyst composition of the invention. There isa balance between the activity and selectivity desired which may resultin a different optimum zeolite content in the carrier depending on theSAR of the zeolite used. Generally a higher zeolite content may in somecases be advantageous to produce a higher activity from the catalystcomposition, while a lower zeolite content may provide a higherselectivity.

While this balance may cause a different optimum depending on theconditions utilized in the ethylbenzene dealkylation process, generallyit is preferred to minimize the amount of zeolite used in the catalystcarrier, since a higher amount of zeolite may negatively affect thephysical properties of the catalyst carrier such as lowering itsstrength. It is generally preferred that the carrier is composed of inthe range of from 30 to 80 wt %, most preferably from 50 to 70 wt %,silica and in the range of from 20 to 70 wt %, most preferably from 30to 50 wt %, zeolite.

A very suitable catalyst carrier for the present invention contains theZSM-5 crystals of the present invention in an amount in the range offrom 20 to 50 wt %, especially 25 to 40 wt %.

Preferably there is no other component than binder, preferably silica,and ZSM-5 zeolite in the carrier. However it is possible to include upto 10 wt % of other components whilst still obtaining the benefits ofthe present invention. Such other components may be selected from otherrefractory inorganic oxide binder materials and other zeolites. Otherbinder materials may be alumina, and magnesia. Examples of otherzeolites are 8, 10, or 12-membered ring zeolites, for example mordenite,and zeolite beta, and acidic mesoporous materials such as the MCM-seriesof zeolites, e.g. MCM-22 and MCM-41.

The carrier is conveniently a shaped carrier and may be treated toenhance the activity of the zeolite component. It may be advantageous toperform a surface modification, such as is described in U.S. Pat. No.6,949,181.

Modification of the molecular sieve reduces the mole percentage ofalumina which basically implies that the number of acid sites isreduced. This can be achieved in various ways. A first way is applying acoating of a low acidity inorganic refractory oxide onto the surface ofthe crystallites of the molecular sieve.

Another very useful way of modifying the molecular sieve is bysubjecting it to a dealumination treatment. In general, dealumination ofthe crystallites of a molecular sieve refers to a treatment, wherebyaluminium atoms are either withdrawn from the molecular sieve frameworkleaving a defect or are withdrawn and replaced by other atoms, such assilicon, titanium, boron, germanium, or zirconium.

In U.S. Pat. No. 5,242,676, a very suitable method for the dealuminationof the surface of zeolite crystallites is disclosed. Another method forobtaining a zeolite having a dealuminated outer surface is disclosed inU.S. Pat. No. 4,088,605.

Of the (surface) dealumination methods described above, the methodinvolving the treatment with a hexafluorosilicate, most suitablyammoniumhexa-fluorosilicate (AHS) as described in U.S. Pat. No.6,949,181, can be expected to offer additional advantage.

The catalyst composition of the invention most suitably also contains ahydrogenation metal selected from platinum, tin, lead, silver, copper,and nickel. Preferably the metal component is platinum. More preferablyan additional metal component selected from tin, lead, copper, nickel,and silver, is present.

The platinum component is preferably present in an amount in the rangeof from 0.001 to 0.1 wt %, based on total catalyst. Preferably theadditional metal component is less than 1 wt %. Most suitably theplatinum content is present in an amount in the range of from 0.01 to0.1 wt %, and preferably from 0.01 to 0.05 wt %. The additional metalcomponent is most suitably present in the range from 0.001 to 0.5 wt %.

An additional metal component of copper, nickel or silver is preferablypresent in the range of from 0.0001 to 0.1 wt %, based on the totalcatalyst. If tin or lead is the additional metal component then it ispresent in an amount in the range of from 0.01 to 0.5 wt %, based ontotal catalyst, most suitably present in an amount in the range of from0.1 to 0.5, preferably 0.2 to 0.5, wt %.

The catalyst composition of the present invention may be prepared usingstandard techniques for combining the zeolite, binder such as silica,and optional other carrier components; shaping; compositing with themetals components; and any subsequent useful process steps such asdrying, calcining, and reducing.

The shaping may be into any convenient form such as powders, extrudates,pills and granules. Preference is given to shaping by extrusion. Toprepare extrudates, commonly the ZSM-5 zeolite will be combined with thebinder, preferably silica, and if necessary a peptizing agent, and mixedto form a dough or thick paste. The peptizing agent may be any materialthat will change the pH of the mixture sufficiently to inducedeagglomeration of the solid particles. Peptising agents are well knownand encompass organic and inorganic acids, such as nitric acid, andalkaline materials such as ammonia, ammonium hydroxide, alkali metalhydroxides, preferably sodium hydroxide and potassium hydroxide, alkaliearth hydroxides and organic amines, e.g. methylamine and ethylamineAmmonia is a preferred peptizing agent and may be provided in anysuitable form, for example via an ammonia precursor. Examples of ammoniaprecursors are ammonium hydroxide and urea. It is also possible for theammonia to be present as part of the silica component, particularlywhere a silica sol is used, though additional ammonia may still beneeded to impart the appropriate pH change.

The amount of ammonia present during extrusion may affect the porestructure of the extrudates which may provide advantageous properties.Suitably the amount of ammonia present during extrusion may be in therange of from 0 to 5 wt % based on total dry mixture, more preferably 0to 3 wt %, preferably 0-1.9 wt % on dry basis. It is preferable that acalcination step be carried out on the resultant extrudate prior toemplacement of the metal components, this is preferably carried out attemperatures above 500° C. and typically above 600° C.

The metals emplacement onto the formed carrier may be by methods usualin the art. The metals can be deposited onto the carrier materials priorto shaping, but it is preferred to deposit them onto a shaped carrier.

Pore volume impregnation of the metals from a metal salt solution is avery suitable method of metals emplacement onto a shaped carrier. Themetal salt solutions may have a pH in the range of from 1 to 12. Theplatinum salts that may conveniently be used are chloroplatinic acid andammonium stabilised platinum salts. The additional silver, nickel orcopper metal salt will typically be added in the form of water solubleorganic or inorganic salt in solution. Examples of suitable salts arenitrates, sulphates, hydroxides and ammonium (amine) complexes. Examplesof suitable tin salts that may be utilized are stannous (II) chloride,stannic (IV) chloride, stannous sulphate, and stannous acetate. Examplesof suitable lead salts are lead acetate, lead nitrate, and leadsulphate.

Where there is more than one metals component, the metals may beimpregnated either sequentially or simultaneously. It is preferable thatthe metals be added simultaneously. Where simultaneous impregnation isutilised the metal salts used must be compatible and not hinder thedeposition of the metals. It may be useful to utilise a complexing agentor chelating agent in a combined bimetallic salt solution to preventunwanted metals precipitation. Examples of suitable complexing agentsare EDTA (ethlyenediaminetetraacetic acid), and derivatives thereof;HEDTA (N-(2-hydroxyethyl)ethylenediamine-N,N′,N′-triacetic acid), EGTA(ethylene glycol-bis(2-aminoethylether)-N,N,N′,N′-tetraacetic acid),DTPA (diethylene tridiamine pentaacetic acid), and NTA (nitrilotriaceticacid). Where EDTA is used, it is conveniently used in a molar ratio tothe additional metal of from 0.1 to 3, especially 1 to 2.

After shaping of the carrier, and also after metals impregnation, thecarrier/catalyst composition is suitably dried, and calcined. Dryingtemperatures are suitably 50 to 200° C.; drying times are suitably from0.5 to 5 hours. Calcination temperatures are very suitably in the rangeof from 200 to 800° C., preferably 300 to 600° C. For calcination of thecarrier, a relatively short time period is required, for example 0.5 to3 hours. For calcination of the catalyst composition, it may benecessary to employ controlled temperature ramping at a low rate ofheating to ensure the optimum dispersion of the metals: such calcinationmay require from 5 to 20 hours.

Prior to use, it is necessary to ensure that any hydrogenation metals onthe catalyst composition are in metallic (and not oxidic) form.Accordingly, it is useful to subject the composition to reducingconditions, which are, for example, heating in a reducing atmosphere,such as in hydrogen optionally diluted with an inert gas, or mixture ofinert gases, such as nitrogen and carbon dioxide, at a temperature inthe range of from 150 to 600° C. for from 0.5 to 5 hours.

The catalyst composition of the invention finds especial use in theselective dealkylation of ethylbenzene.

The ethylbenzene feedstock most suitably originates directly from areforming unit or naphtha pyrolysis unit or is the effluent of a xyleneisomerisation unit. Such feedstock usually comprises C₇ to C₉hydrocarbons, and in particular one or more of o-xylene, m-xylene,p-xylene, toluene, and benzene in addition to ethylbenzene. Generallythe amount of ethylbenzene in the feedstock is in the range of from 0.1to 50 wt % and the total xylene content is typically at least 20 wt %.Typically the xylenes will not be in a thermodynamic equilibrium, andthe content of p-xylene will accordingly be lower than that of the otherisomers.

The feedstock is contacted with the catalyst composition in the presenceof hydrogen. This may be carried out in a fixed bed system, a moving bedsystem, or a fluidized bed system. Such systems may be operatedcontinuously or in batch fashion. Preference is given to continuousoperation in a fixed bed system. The catalyst may be used in one reactoror in several separate reactors in series or operated in a swing systemto ensure continuous operation during catalyst change-out.

The process is suitably carried out at a temperature in the range offrom 300 to 500° C., a pressure in the range of from 0.1 to 50 bar (10to 5,000 kPa), using a liquid hourly space velocity of in the range offrom 0.5 to 20 h⁻¹. A partial pressure of hydrogen in the range of from0.05 to 30 bar (5 to 3,000 kPa) is generally used. The feed to hydrogenmolar ratio is in the range of from 0.5 to 100, generally from 1 to 10mol/mol.

The following Examples illustrate the present invention.

EXAMPLES Example 1

The following matrix of silica to alumina ratios was obtained from theelemental analysis carried out by SEM/EDX by the procedures hereinbeforedescribed on a ZSM-5 sample (Sample A) which was synthesized followingthe procedure of Iwayama et al in U.S. Pat. No. 4,511,547. Synthesisdetails such as crystallization temperature, time, etc. are given inTable 1 below, together with physical property data for the preparedsample. The ZSM-5 zeolite had an overall bulk silica to alumina ratio of43.

The gray shaded area was used for the generation of a narrow slit linescan profile (by averaging the values vertically). The obtained profileis shown graphically in FIG. 1.

The sample shows a U shaped profile, where the silica to alumina ratiois higher at the edges of the crystallite than at the centre.

Example 2

In another synthesis, carried out analogously to Example 1, a new batchof ZSM-5 zeolite was prepared (Sample G). Synthesis and analysis detailsare given in the Table below. This zeolite also had an overall bulksilica to alumina ratio of 46.

The obtained SAR map is shown below:

The narrow slit line scan profile obtained from the gray area is shownin FIG. 2.

This sample shows a profile, where the silica to alumina ratio hardlychanges from the edge to the centre of the crystallite.

Example 3

Additional ZSM-5 zeolite syntheses were performed following the Iwayamamethod as in Example 1, with slight variations in the crystallizationtemperature and the agitation speed. Table 1 below summarizes thesynthesis conditions, the bulk analysis results, and provides additionalinformation. Surface area was measured by the B.E.T. method.

TABLE 1 Catalyst description Sample A Sample B Sample C Sample D SampleE Sample F Sample G Sample H Crystallinity (% vs. ref.) 109 105  95+*117 104 106  93+* n.a. SiO₂/Al₂O₃ 43.1 43.2   44.6 44.1 42.5 41.7   46.2n.a. Na₂O (%) 0.01 0.01    0.01 0.01 0.02 0.05    0.03 n.a. Surface area(m².g⁻¹) 420 440 404  447 406 434 389  n.a. Autoclave scale (gal. 30 3030 30 30 30 30 n.a. Tartaric acid source** lab-L lab-L com-DL com-DLcom-DL com-L com-L com-L Crytallization T (° C.) 180 170 180  180 180180 170  180 Time at T (h) 24 48 24 24 24 24 48  24 Agitator speed (rpm)60 30 35 60 35 35 35 n.a. *In these samples magadiite was found.**lab-L: laboratory grade L-isomer, com-M: commercial grade mixedisomer, com-L: commercial grade L-isomer of tartaric acid.

Example 4

Catalysts were prepared from some of the zeolite samples prepared inExamples 1, 2 and 3 by mixing the ZSM-5 zeolite with silica as binder,extruding to form a shaped carrier, and then impregnating withhydrogenation metal by pore volume impregnation. Each carrier contained40 wt % zeolite bound with silica (a mixture of Sipernat 50 from Degussaand Bindzil silica sol from EKA Chemicals in a weight ratio of 2:1).Each carrier was impregnated with a Pt/Sn solution so that the finalcatalyst had a composition of 0.02 wt % Pt and 0.4 wt % Sn.

The catalysts were subjected to a catalytic test that mimics typicalindustrial application conditions for ethylbenzene dealkylation. Thisactivity test uses an industrial feed of European origin. Thecomposition of the feed used here is summarized in Table 2.

TABLE 2 Composition of the feed used in the activity testing Feedcomposition EB wt % 13.68 pX wt % 0.18 oX wt % 18.12 mX wt % 62.06toluene wt % 0.48 benzene wt % 0.13 C₇-C₈-naphthenes wt % 5.35 C₉ ⁺aromatics wt % 0.00 Total wt % 100.00 C₈ aromatics sum 94.97 EB in C₈aromatics feed wt % 11.25 pX in xylenes in feed wt % 0.22 oX in xylenesin feed wt % 22.54 mX in xylenes in feed wt % 77.23

The activity test is performed once the catalyst is in its reducedstate, which is achieved by exposing the dried and calcined catalyst toatmospheric hydrogen (>99% purity) at 450° C. for 1 hour.

After reduction the reactor is pressurized without a cooling step, andthe feed is introduced. This step contributes to enhanced catalystaging, and therefore allows comparison of the catalytic performance atstable operation.

The catalytic datapoints are collected at a condition that exaggeratesthe potential negative operational effects. Therefore, the performanceis measured not at the ideal industrial operating condition(s), but atthose that allow a better differentiation of the various performanceparameters used to evaluate catalysts in this application.

In the present case, a weight hourly space velocity of 4.6 h⁻¹, ahydrogen to feed ratio of 2.5 mol·mol⁻¹, a total system pressure of 1.3MPa was used. The temperature was varied between 360 and 410° C. toachieve the required conversion for easier comparison.

The performance characteristics evaluated in this test are as follows:

${{EB}\mspace{14mu} {conversion}\mspace{14mu} \left( {{wt}\mspace{14mu} \%} \right)} = {\frac{{EB}_{f.} - {EB}_{{pr}.}}{{EB}_{f.}} \times 100}$${{Xylene}\mspace{14mu} {losses}\mspace{14mu} \left( {{wt}\mspace{14mu} \%} \right)} = {\frac{{Xyl}_{f.} - {Xyl}_{{pr}.}}{{Xyl}_{f.}} \times 100}$

where EB stands for ethylbenzene, Xyl for xylenes in general (allisomers), f for feed, and pr for product.

Example 5

Samples A, B, D, E, F, G and H prepared in Examples 1, 2 and 3 wereanalysed as described in Example 1 and 2. Typically 5 crystallites wereselected randomly in the electron microscope, except for Sample B whereonly 3 crystallites were measured. These selected crystallites weresubjected to the hereinbefore described X-ray mapping.

From the X-ray maps, the narrow slit line scan profiles were generated,and the slope of the profiles determined. These slopes were averaged perzeolite synthesis batch, and plotted against the xylene losses obtainedin the test of Example 4 using the same zeolites in the catalysts.

The ratio of the average SARs (edge to centre) of the crystallites foreach sample is given in Table 3 below and the plot of profile slopeagainst xylene losses is shown in FIG. 3.

TABLE 3 Synthesis SAR ratio of the local averages Sample at edges andcentre Sample A 1.40 Sample B 1.17 Sample D 1.14 Sample E 1.06 Sample F1.23 Sample G 1.08 Sample H 1.28The most preferred materials are Sample A and Sample H in terms ofhaving the lowest xylene losses.

An unexpected good correlation could be found between the slope of thesilica to alumina distribution and the xylene losses at 65 wt %ethylbenzene conversion. This is to say that the steeper the profile,i.e. the larger the difference between the aluminium concentrationbetween the centre and the edges of the zeolite crytallites, the lowerthe xylene losses in the ethylbenzene dealkylation process.

That which is claimed is:
 1. ZSM-5 crystals, comprising a ZSM-5crystallite having an average SAR at the edge of the crystallite and anaverage SAR at the centre of the crystallite, as determined via SEM/EDXor TEM/EDX elemental analysis, wherein the average SAR at the edge ofthe crystallite is higher than the average SAR at the centre of thecrystallite.
 2. ZSM-5 crystals as claimed in claim 1, wherein the ZSM-5crystallite has a ratio of the average SAR at the edge of thecrystallite to the average SAR at the centre of the crystallite that isat least 1.15.
 3. ZSM-5 crystals as claimed in claim 1, wherein theratio is at most
 3. 4. ZSM-5 crystals as claimed in claim 1, wherein theslope of a second order polynomial fitted to the SAR values between thetwo edge SAR maxima, expressed as −y′₀, is at least
 2. 5. ZSM-5 crystalsas claimed in claim 1, wherein the slope of a second order polynomialfitted to the SAR values between the two edge SAR maxima, expressed as−y′₀, is at most
 6. 6. A catalyst composition, comprising: ZSM-5crystals, comprising a ZSM-5 crystallite having an average SAR at theedge of the crystallite and an average SAR at the centre of thecrystallite, as determined via SEM/EDX or TEM/EDX elemental analysis;and a silica binder in an amount in the range of from 30 to 80 wt %,based on the total of ZSM-5, crystals.
 7. A catalyst composition asclaimed in claim 6, which also contains a hydrogenation metal selectedfrom the group consisting of platinum, tin, lead, silver, copper, andnickel.
 8. A catalyst composition as claimed in claim 7, wherein thetotal amount of ZSM-5 crystals present in the catalyst composition is inthe range of from 20 to 50 based on the total carrier.
 9. A catalystcomposition as claimed in claim 8, wherein the catalyst carrier has beensubjected to a dealumination treatment with ammonium hexafluorosilicate.